The present invention relates in general to coupling devices and more specifically refers to a control device for a coupling electromagnet which can be associated with a starter motor used for starting an internal combustion engine.
As is known the use of electric motors for starting heat engines, in particular internal combustion engines is widely diffused. In the case of internal combustion engines of motor vehicles this starting system has by now in fact become standard.
To start an internal combustion engine by means of an electric starter motor the motor and the engine are coupled by means of gear wheels. On the drive shaft of the starter motor there is fitted a gear wheel commonly called a pinion, whilst on the internal combustion engine's crankshaft there is fitted another gear wheel, called a ring gear, having a decidedly greater diameter than the diameter of the pinion.
By energising the starter motor this, by means of the pinion and ring gear which mesh together, drive the internal combustion engine's crankshaft allowing the engine to start. It is, however, evident that the pinion and ring gear cannot be permanently in mesh with one another. In fact, if this were to happen, once the internal combustion engine had started, it would drive the starter motor at high speed certainly causing damage to the two gear wheels and/or to the starter motor. For this purpose the starter motor is therefore provided with an electromagnet intended to cause engagement of the pinion, which can slide in an axial direction with respect to the ring gear in such a way that the respective teeth only mesh during the starting operation.
This system, although accepted and universally adopted in the motor vehicle sector, is not however free from disadvantages. In fact, the conventional starting systems do not provide any control for the supply of the electromagnets so that the pinion and ring gear are subject to high stresses due to the excessive speed with which the pinion comes into contact with the ring gear. This excessive speed also causes an annoying acoustic noise especially if the teeth of the pinion strike against those of the ring gear. Moreover, since the internal combustion engine tends always to stop in predetermined positions the teeth of the ring gear involved in these impacts tend always to be the same thereby causing localised wear.
Further problems can arise for example if the user, when starting the engine, maintains the starting contacts closed for a period of time greater than necessary, thereby causing excessive wear and overheating of the starter motor.
Some solutions proposed to overcome these disadvantages are known in the art. For example the document EP-A-0 727 577 describes a starter system comprising a device for controlling the speed of translation of the coupling electromagnet using a tachometric sensor for the purpose of detecting this speed of translation. In the document EP-A-0 727 667, there is described an electromagnetic tachometric sensor which can be used in such a starter system.
This arrangement allows an effective control of the speed of translation of the coupling electromagnet to be effected, but is not free from disadvantages. In fact, the use of a tachometric sensor involves a not insignificant increase in the cost and complexity of the system. There are also known arrangements in which this control is effected without a tachometric sensor but by measuring the current in the winding of the coupling electromagnet. For example, in the document U.S. Pat. No. 5 383 428 there is described a starter system comprising an electronic control unit operable to detect the current in the winding of the coupling electromagnet by means of a measurement resistor (or shunt) and to control this current. This measurement resistor is connected to the winding of the coupling electromagnet in that it is constituted by a portion of copper wire constituting a part of this winding. This arrangement however has the disadvantage of not allowing a sufficiently accurate control of the speed of translation of the electromagnet for various reasons which will be discussed in more detail hereinafter.